1. Technical Field
The present disclosure relates to a drive system for a hybrid vehicle including an engine and motors as driving force sources, and more particularly to a drive system for a hybrid vehicle including two motors or motor-generators in addition to the engine.
2. Description of Related Art
A drive system for a hybrid vehicle is known which includes an engine, first motor, second motor, speed change mechanism, and a power split mechanism coupled between a speed change part and an output part located closer to drive wheels (see, for example, Internal Publication No. 2013/114594 (WO 2013/114594)). The speed change mechanism includes a clutch mechanism, a brake mechanism, and a first planetary gear mechanism, for example, and increases or reduces and delivers torque generated in the engine. The power split mechanism includes a second planetary gear mechanism, for example, and splits driving force received from the speed change mechanism, to the first motor side and the output side.
The drive system can be placed in an HV (Hybrid Vehicle) mode and an EV (Electric Vehicle) mode, by changing engagement states of the clutch mechanism and the brake mechanism. In the HV mode, the vehicle travels using driving force of both of the engine and the motor(s). The HV mode includes a “HIGH” mode and a “LOW” mode as “differential” modes. In the “HIGH” mode, a high-speed gear position is set in which the speed ratio as the ratio of the rotational speeds of the engine and an output element of the speed change mechanism is smaller than the speed ratio of “1:1” representing direct coupling. In the “LOW” mode, a low-speed gear position, which is a direct-coupling position, is set. The EV mode includes a single-motor mode in which the vehicle travels using driving force produced by the second motor, and a both-motor mode in which the vehicle travels using driving force of both of the first motor and the second motor.
Another example of drive system is known in which a speed change part is coupled between a power split mechanism and an output part, and the speed change part increases or reduces driving force obtained by splitting driving force produced by an engine, at a given speed ratio, and directly delivers it to the output part (see, for example, Japanese Patent Application Publication No. 2014-51146 (JP 2014-51146 A)). A first motor included in this type of drive system is a motor that controls the engine speed, and the engine and the first motor are respectively coupled to certain rotating elements of a planetary gear mechanism that constitutes the power split mechanism. The driving force split by the power split mechanism and distributed to the output side is delivered to the speed change part. The speed change part, which changes torque delivered from an output element of the power split mechanism, is provided by a planetary gear mechanism having an input element to which the output element of the power split mechanism is coupled, a reaction-force element, and an output element. The speed change part also includes a brake that selectively fixes the reaction-force element, and a clutch that couples the reaction-force element with the input element, so as to selectively integrate the whole speed change unit.
However, in the drive system in which the power split mechanism is coupled between the speed change part and the output part, it is impossible to set a traveling mode for increasing or reducing and delivering the driving force split by the power split mechanism and distributed to the output side (driving force directly transmitted from the engine toward drive wheels), at different speed ratios. Thus, there is room for improvement in terms of the variety of traveling modes.
Also, in the drive system in which the speed change part is coupled between the power split mechanism and the output part, it is necessary for the first motor to generate reaction-force torque when the engine produces driving force; therefore, the first motor cannot function as a motor that produces torque for traveling the vehicle. Namely, in this type of drive system, a both-motor drive mode cannot be set in which both of the first motor and the second motor deliver driving force for traveling the vehicle, to the output part.